To better understand
Bio-Mat, let’s review what happens when septic effluent
is introduced into soil using gravity distribution.
Effluent flows from the septic tank into the soil treatment
trench through a number of holes that are ½ inch
in diameter in the distribution pipe. Then through the distribution
media (rock in our case) to the soil surface. When the effluent
reaches the soil a condition called biomat is created. The
biomat is formed by anaerobic bacteria in the effluent and
any finely sized organic suspended solids carried over from
the septic tank.
The bacteria secretes a sticky substance around the outside
soil and rock particles. The biomat develops along the trench
bottom and ponds the effluent in the trench. As the liquid
rises, biomat develops along the sidewalls.
The main result of biomat formation is dramatic slowing
of the infiltration rate of the effluent into the soil,
creating unsaturated flow conditions. This is ideal for
growth of aerobic bacteria and other soil organisms that
help with treatment of pathogens and other contaminants.
THIS IS GOOD NEWS. This explains why the biomat--- when
properly managed---is a necessary component of soil treatment
capabilities. In addition the biomat reaches equilibrium.
If effluent quality is maintained, the biomat will have
the same thickness and permeability overtime.
This condition is referred to as the Long Term Acceptance
Rate, or LTAR. The LTAR is related to soil texture class,
soil structure, and consistency to predict the loading rates
in soil where the biomat is fully developed. Research on
these relationships has been conducted since the early 1970’s
and in fact, the condition was recognized as early as the
Sandy Soils are Different
These relationships are reflected and used in almost all
current codes dictating the design and installation of onsite
treatment systems. Sandy soils are the one exception.
Since the sandy soils particle size is larger (0.025-2.0mm)
the size of the pores are also larger, allowing effluent
to move more rapidly into and through the sand, often without
forming a biomat. (Remember with rock it slows down the
movement of effluent). This rapid movement does not allow
time for treatment. So in the early ‘70’s and
again as recently as 2006, column studies were conducted
looking at virus removal in sands under different loading
These studies showed good virus removal in 2 feet of sand
if the loading rate does not exceed 1.2 gallons per square
foot/day. This has been confirmed by the state of Florida.
This is the loading number for sands found in most of our
One unexpected result of this study showed a thin biomat
formed in the soil and this biomat was very effective at
reducing the flow rate into the soil. Over several years
of study, the biomat showed an acceptance rate of 0.6-gallons/square
foot/day. This was confirmed over the next several years
and the loading rate was incorporated into the state code.
Remember we are talking about a rock media in the drainfield,
which Dixie Septic Tank Inc. uses. Our competition does
not use rock or any media other than sand.
Another interesting note on how sandy soils react to septic
tank effluent: If you conduct a percolation test, the rate
would be in the range of 30 seconds to 3 minutes per inch:
so you would not distinguish these soils from other types
of sands on the basis of percolation rates.
The Bottom Line
So what about the treatment approaches in these soils?
For the coarse and medium sandy soils, the only way to ensure
the loading rate doesn’t exceed 1.2 gallons/square
foot/day is to spread the effluent out evenly over the entire
soil treatment area. “HALLELUJAH” This is exactly
what Dixie Septic Tank does with our “Equal Distribution™”
system, using our Patented Pipe, and using recycled concrete
as the drainfield media.
So if you are looking for a great system that will last
a long, long time, give us a call, we are more than willing
to help. (386) 738-3030.